Fix deleteIfNewAndEmpty() being called after annotation was destroyed

Listen for the BEFORE_ANNOTATION_CREATED event on the <annotation>'s own
scope which is destroyed when the component itself is destroyed, rather
than the root scope which lives for the lifetime of the application.

h/static/scripts/directive/annotation.js

@@ -124,7 +124,7 @@ function AnnotationController(
 
     // When a new annotation is created, remove any existing annotations that
     // are empty
-    $rootScope.$on(events.BEFORE_ANNOTATION_CREATED, deleteIfNewAndEmpty);
+    $scope.$on(events.BEFORE_ANNOTATION_CREATED, deleteIfNewAndEmpty);
 
     // Call `onGroupFocused()` whenever the currently-focused group changes.
     $scope.$on(events.GROUP_FOCUSED, onGroupFocused);